1. Field of the Invention
The present invention relates to implementation of "formant filtering" in a computor organ by the selective attentuation or elimination of certain constituent Fourier components from the waveshape synthesis.
2. Related Patent
This application is related to the inventor's U.S. Pat. No. 3,809,786 issued May 7, 1974, entitled COMPUTOR ORGAN.
3. Description of the Prior Art
In most electronic musical instruments, a musical signal having a complete tonal spectrum is generated. Then, to achieve special effects, filters are used selectively to attentuate or eliminate certain frequency bands. For example, a band pass filter will permit all frequencies in the passband to be reproduced by the sound system, but will block signals of higher or lower frequency.
The use of such filtering in electronic musical synthesis has important applications. For example, most orchestral musical instruments exhibit tone formants. That is, a different tonal spectrum is produced for notes of different fundamental frequency. Further, the tones in certain frequency ranges may be selectively enhanced or attenuated. In a brass instrument, the different tube lengths and curvatures used for different notes causes some notes to contain accentuated harmonic components, while for other notes the same order harmonics are attenuated. Formant filters are used to synthesize such effects in electronic musical instruments.
The basic COMPUTOR ORGAN disclosed in the above referenced U.S. patent is a "sliding-formant" system in which the harmonic spectrum ideally is exactly the same for each note on the keyboard. Conventional analog frequency domain or digital time domain filters could be employed with the computor organ to limit the band of frequencies supplied to the sound system, and thereby to implement formant filtering. However, the computor organ is unique in that each individual constituent Fourier component of every note is evaluated independently. Consequently, the equivalent of formant filtering can be achieved by modifying the strength of certain Fourier components during note generation.
Thus, a principal object of the present invention is the implementation of "formant filtering" by the selective attenuation or inhibition of Fourier components in certain frequency bands during the initial tone generation, without the use of actual filters. In other words, "formant filtering" is achieved by attenuating, accentuating or eliminating from the original musical waveshape synthesis those Fourier components which lie within the undesired frequency bands. This is in contrast with the prior art approach in which a complete tonal spectrum first is generated as a waveform, and subsequently actual filters are used to attenuate or eliminate selected frequency bands.
A wide variety of special effects can be achieved using formant filtering, particularly by varying the filter characteristics as a function of time. As an example, this technique can be used to implement a "wah-wah" effect. Here, the formant filter passband is swept through the harmonic spectrum of the produced note. Typically, lower frequency components first are accentuated, followed by sliding accentuation of gradually higher frequency harmonics. This can be used to imitate a trumpet player covering and uncovering the trumpet bell with a mute.
In the past, "wah-wah" has been implemented by generating a reedtype tone which then is processed by a formant filter having a passband that can be varied electronically, e.g., in response to a foot-controlled switch. To eliminate the difficulty of coordination required to produce foot-controlled "wah-wah", some systems automatically cause the formant filter to sweep from low to high frequency, at a selectable rate, each time that a note is played. As in the general formant filtering case, such prior art "wah-wah" or other time-varying filter effects all have been produced by generating a note having a full tonal spectrum, and subsequently filtering out the selected formants. Another object of the present invention is to accomplish time-variant "formant filtering" in a computor organ by selective, time-variant attenuation or elimination of constituent Fourier components from the waveshape synthesis.
Other unique effects are achieved by accentuating certain frequencies within the formant filter passband. This accentuation may be restricted to a relatively narrow frequency range, corresponding to the resonance frequency of a fairly high Q filter. Hence a further object of this invention is to implement such "Q-resonance" accentuation in a computor organ. The Q-resonance frequency itself may be time variant.